1. Field of the Invention
The present invention relates to control method and apparatus for controlling a machine tool feed system. More specifically, the invention relates to control method and control apparatus capable of effectively suppressing vibrations that occur in the feed system.
2. Description of the Prior Art
Typically, a feed system for an NC machine tool is controlled based on an operation command signal generated according to an NC program as well as a current position signal and a current speed signal fed back from the feed system. In more detail, an operation command signal is generated according to an NC program, and a speed command signal is generated by multiplying a deviation between the generated operation command signal and a current position signal fed back from the feed system by a positional loop gain. Then, an electric-current command signal is generated by multiplying a deviation between the generated speed command signal and a current speed signal fed back from the feed system by a speed loop gain, and the drive motor of the feed system is controlled based on the generated electric-current command signal.
Since the speed loop gain largely affects the responsivity of the feed system, there is a need for increasing the speed loop gain in order to implement a high-speed, high-precision feed system. On the other hand, excessively increasing the speed loop gain would cause the control system to be unstable, which may lead to an occurrence of resonance around the natural frequency of the feed system or the whole machine tool.
For this reason, it has conventionally been practiced that while the speed loop gain is increased, vibrations of the feed system are suppressed by eliminating frequency components at and around the natural frequency of the feed system or the whole machine tool out of vibrational components contained in the electric-current command signal by means of a band elimination filter. More specifically, vibrations of the feed system would cause vibrational frequency components to be added to the current speed signal fed back from the feed system, so that the vibrational frequency components would be contained in the electric-current command signal computed based on the current speed signal. Therefore, eliminating frequency components at and around the natural frequency of the feed system or the whole machine tool out of the frequency components contained in the electric-current command signal makes it possible to prevent occurrence of a resonance state even if the vibrations of the feed system or the whole machine tool become larger.
Further, the value of the speed loop gain and the elimination band of the band elimination filter are so set in the manufacturing process of the machine tool that the feed system comes to an optimum operating state.
In this connection, rigidity of machine tools does not keep constant from their manufacture to the future, but varies depending on the running conditions of the machine tools or with a lapse of years. Accordingly, because of such changes in rigidity of machine tools, the value of the speed loop gain and the elimination band of the band elimination filter that have been set to their optimums during the manufacture are not necessarily the optimum ones at the current time point.
For this reason, it has been the case that vibrations would occur to the feed system due to changes in the rigidity of the machine tool, causing deteriorations of machining precision such as a deterioration of surface roughness, disadvantageously. Besides, larger vibrations would lead to a resonance state of the feed system or the whole machine tool, which may incur a serious situation.
The present invention having been accomplished in view of these and other problems, an object of the present invention is to provide control method and control apparatus for a machine tool feed system capable of preventing vibrations of the feed system and thereby maintaining its optimum operating state for a long run regardless of variations in the rigidity of the machine tool.
In order to achieve the above object, according to the present invention, there is provided a method for controlling a feed system of an NC machine tool, comprising the steps of: generating an operation command signal according to an NC program; generating a speed command signal by multiplying a deviation between the generated operation command signal and a current position signal fed back from the feed system by a positional loop gain; generating an electric-current command signal by multiplying a deviation between the generated speed command signal and a current speed signal fed back from the feed system by a speed loop gain; and controlling a drive motor for the feed system based on the generated electric-current command signal, the method further comprising the steps of:
detecting vibration level and vibrational frequency of the generated electric-current command signal; and adjusting a value of the speed loop gain in response to the detected vibration level and vibrational frequency.
This control method can be embodied preferably by the following control apparatus. That is, the control apparatus comprises: an operation command generating section for generating an operation command signal according to an NC program; a position controlling section for generating a speed command signal by multiplying a deviation between the operation command signal generated in the operation command generating section and a current position signal fed back from the feed system by a positional loop gain; a speed controlling section for generating an electric-current command signal by multiplying a deviation between the speed command signal generated in the position controlling section and a current speed signal fed back from the feed system by a speed loop gain; and adjustment means for detecting vibration level and vibrational frequency of the electric-current command signal generated in the speed controlling section, and adjusting a value of the speed loop gain in response to the detected vibration level and vibrational frequency, wherein the control apparatus controls a drive motor for the feed system of the NC machine tool based on the electric-current command signal generated in the speed controlling section.
In this invention, first, in the operation command generating section, an operation command signal is generated according to an NC program. Then, in the position controlling section, a speed command signal is generated by multiplying a deviation between the operation command signal generated in the operation command generating section and a current position signal fed back from the feed system by a positional loop gain. Further, in the speed controlling section, an electric-current command signal is generated by multiplying a deviation between the speed command signal generated in the position controlling section and a current speed signal fed back from the feed system by a speed loop gain. Thus, the drive motor for the feed system is controlled based on the generated electric-current command signal.
Furthermore, vibration level and vibrational frequency of the electric-current command signal generated in the speed controlling section are detected by the adjustment means. The adjustment means adjusts the value of the speed loop gain to be used by the speed controlling section in response to the detected vibration level and vibrational frequency.
As described before, excessively increasing the speed loop gain would cause the control system to be unstable, causing the feed system to vibrate. Vibrations of the feed system would cause the vibrational frequency components to be added to the current speed signal fed back from the feed system, so that the vibrational frequency components would be contained in the electric-current command signal computed based on the current speed signal. The adjustment means detects the vibration level and frequency contained in the electric-current command signal in this way, where if the detected vibration level has come beyond a certain level, and/or if the vibrational frequency is at and around the natural frequency of the feed system or the whole machine tool, the adjustment means lowers the value of the speed loop gain to be used by the speed controlling section. As a result of this, it becomes possible to prevent deteriorations of machining precision such as surface roughness due to excessive vibrations of the feed system beyond a certain level, or to prevent the feed system or the whole machine tool from going into a resonance state due to the vibrations of the feed system.
Also, according to the present invention, there is provided a method for controlling a feed system of an NC machine tool, comprising the steps of: generating an operation command signal according to an NC program; generating a speed command signal by multiplying a deviation between the generated operation command signal and a current position signal fed back from the feed system by a positional loop gain; generating an electric-current command signal by multiplying a deviation between the generated speed command signal and a current speed signal fed back from the feed system by a speed loop gain; and controlling a drive motor for the feed system based on an electric-current command signal obtained by eliminating frequency components in a predetermined frequency band from the generated electric-current command signal, the method further comprising the steps of:
detecting vibration level and vibrational frequency of the electric-current command signal obtained by eliminating frequency components in the predetermined band; and adjusting the band of frequency components to be eliminated from the electric-current command signal in response to the detected vibration level and vibrational frequency.
This control method can be embodied preferably by the following control apparatus. That is, the control apparatus comprises: an operation command generating section for generating an operation command signal according to an NC program; a position controlling section for generating a speed command signal by multiplying a deviation between the operation command signal generated in the operation command generating section and a current position signal fed back from the feed system by a positional loop gain; a speed controlling section for generating an electric-current command signal by multiplying a deviation between the speed command signal generated in the position controlling section and a current speed signal fed back from the feed system by a speed loop gain; a filtering section for eliminating frequency components in a predetermined frequency band from the electric-current command signal generated in the speed controlling section; and adjustment means for detecting vibration level and vibrational frequency of the electric-current command signal processed by the filtering section, and adjusting the band of frequency components to be eliminated from the electric-current command signal by the filtering section in response to the detected vibration level and vibrational frequency, wherein the control apparatus controls a drive motor for the feed system based on an electric-current command signal processed by the filtering section.
In this invention, as in the foregoing case, first, in the operation command generating section, an operation command signal is generated according to an NC program. Then, in the position controlling section, a speed command signal is generated by multiplying a deviation between the operation command signal generated in the operation command generating section and a current position signal fed back from the feed system by a positional loop gain. Further, in the speed controlling section, an electric-current command signal is generated by multiplying a deviation between the speed command signal generated in the position controlling section and a current speed signal fed back from the feed system by a speed loop gain.
Subsequently in the filtering section, the electric-current command signal generated in the speed controlling section has its frequency components in a predetermined frequency band eliminated. Thus, the drive motor for the feed system is controlled based on the electric-current command signal that has been subjected to the elimination process.
As described before, whereas the speed loop gain needs to be increased in order to implement high speed and high precision of the feed system, excessively increasing the speed loop gain would cause the control system to be unstable, which would in turn cause the feed system to vibrate. Vibrations of the feed system would cause the vibrational frequency components to be added to the current speed signal fed back from the feed system, so that the vibrational frequency components would be contained in the electric-current command signal computed based on the current speed signal. The vibrations of the feed system, with the frequencies at and around the natural frequency of the feed system or the whole machine tool, would result in a resonance of the feed system or the whole machine tool. The filtering section eliminates frequency components at and around the natural frequency of the feed system or the whole machine tool out of the frequency components contained in the electric-current command signal, thus fulfilling a role of preventing the feed system or the whole machine tool from resulting in a resonance state.
Furthermore, the electric-current command signal, from which the frequency components in the predetermined frequency band have been eliminated in the filtering section, has its vibration level and vibrational frequency detected by the adjustment means. In response to the detected vibration level and vibrational frequency, the adjustment means adjusts the band of frequency components to be eliminated from the electric-current command signal in the filtering section.
The elimination band of the filtering section is set during the manufacturing process of the machine tool so that its feed system comes to an optimum operating state. However, rigidity of machine tools does not keep constant from their manufacture to the future, but varies depending on the running conditions of the machine tools or with a lapse of years. Accordingly, because of such changes in rigidity of machine tools, the elimination band that has been set to an optimum during the manufacture is not necessarily the optimum one at the current time point.
According to this invention, vibration level and vibrational frequency of the electric-current command signal that has been processed by the filtering section are detected by the adjustment means, and further, the elimination frequency band for the filtering section is adjusted in response to the detected vibration level and vibrational frequency. Therefore, even if vibrations occurring to the feed system have varied depending on the running conditions of the machine tool or with a lapse of years, adjusting the band of frequency components to be eliminated makes it possible to properly eliminate the frequency components that cause the vibrations, and to prevent the feed system from increasing in vibrations beyond a certain level. As a result of this, it becomes possible to prevent deteriorations of machining precision such as surface roughness due to excessive vibrations, so that the machine tool can be maintained in its optimum operating state over a prolonged period.
In the above-described control method and control apparatus, the adjustment means may be designed so as to detect vibration level and vibrational frequency of the electric-current command signal processed by the filtering section, and to adjust the band of frequency components to be eliminated from the electric-current command signal as well as the value of the speed loop gain, in response to the detected vibration level and vibrational frequency.
In this case, vibration level and vibrational frequency of the electric-current command signal, from which frequency components in a predetermined frequency band have been eliminated by the filtering section, are detected by the adjustment means, and the speed loop gain to be used in the speed controlling section as well as the band of frequency components to be eliminated in the filtering section are adjusted in response to the detecting vibration level and vibrational frequency.
Then, upon occurrence of vibrations in the feed system, when the vibration level has come beyond a certain level or the vibrational frequency has changed due to running conditions of the machine tool or with a lapse of years, the speed loop gain to be used by the speed controlling section or the band of frequency components to be eliminated by the filtering section is adjusted in response to the state of such a change. Therefore, it becomes possible to prevent the feed system from increasing in vibrations beyond a certain level and to prevent deteriorations of machining precision such as surface roughness due to excessive vibrations, while the machine tool can be maintained in its optimum operating state over a prolonged period.